The present invention relates to memory devices. In particular, the present invention relates to methods for manufacturing and structures for programmable resistance memory devices that include a metal oxide.
Resistive RAM (RRAM) is emerging technology for non-volatile memory devices. Some RRAM technologies are characterized by simple cell structures that are scalable, and suitable for use in 3D arrays.
Some RRAM technologies are based on metal oxide memory materials, typically transition metal oxides, which can be caused to change resistance between two or more stable ranges by application of electrical pulses at levels suitable for implementation in integrated circuits, and the resistance can be read and written with random access to indicate stored data.
NiO, TiO2, HfO2, and ZrO2 based RRAM have been investigated for use as memory material in memory cells. See, Baek, et al., “Highly Scalable Non-Volatile Resistive Memory using Simple Binary Oxide Driven by Asymmetric Unipolar Voltage Pulses,” IEDM Technical Digest pp. 23.6.1-23.6.4, IEEE International Electron Devices Meeting 2004. These memory cells are formed by a non-self-aligned process in an M-I-M structure, where M is a noble metal acting as an electrode and “I” is one of NiO, TiO2, HfO2, and ZrO2. This MIM structure requires several additional masks and patterning to form the noble metal electrodes and the memory material, and results in a relatively large memory cell size.
CuxO based RRAM has also been investigated for use as a memory material in memory cells. See, Chen et al., “Non-Volatile Resistive Switching for Advanced Memory Applications,” IEDM Technical Digest pp. 746-749, IEEE International Electron Devices Meeting 2005. The CuxO material is formed by thermal oxidation of a copper via which acts as the bottom electrode for the memory cell, while the top electrode consists of a bi-layer Ti/TiN film that is deposited and etched. This structure requires several additional masks to form the top and bottom electrodes, and results in a relatively large memory cell size. Chen et al. disclose that having a copper bottom electrode complicates erasing of the memory cell since the applied field during erasing may push copper ions into the CuxO. Additionally, CuxO has a relatively small resistance window of 10x.
Cu—WO3 based RRAM has also been investigated for use as a memory material in programmable metallization memory cells. See, Kozicki et al., “A Low-Power Nonvolatile Switching Element Based on Copper-Tungsten Oxide Solid Electrolyte,” IEEE Transactions on Nanotechnology pp. 535-544, Vol. 5, No. 5, September 2006. Switching elements fabricated using tungsten metal, a solid electrolyte based on tungsten-oxide and photo-diffused copper, and a copper top electrode are disclosed. The switching element is formed by tungsten-oxide grown or deposited on tungsten material, a layer of Cu formed on the tungsten-oxide and the Cu photo-diffused into the tungsten-oxide to form the solid electrolyte, and a Cu layer is formed and patterned over the solid electrolyte to act as a top electrode. The switching element changes resistance by applying a bias voltage to cause electro-deposition of Cu ions from the top electrode into the solid electrolyte, and states that “a lack of Cu in the top electrode results in no measurable switching activity” (see page 539, column 1). This structure thus needs a Cu top electrode, involves several process steps to form the solid electrolyte, and necessitates bias voltages of opposite polarities to cause the injection of Cu ions to program and erase the solid electrolyte.
RRAM based on tungsten oxide memory material is particularly appealing as tungsten is commonly used as a conductor material in device fabrication. See for example, U.S. Pat. No. 7,943,920, entitled RESISTIVE MEMORY STRUCTURE WITH BUFFER LAYER, issued on 17 May 2011.